Innate immune responses are dictated by a panel of pathogen recognition receptors, downstream signaling from the receptors and the stimulated activities of various effector molecules. IRF8 is known as an interferon (IFN)-responsive transcription factor that plays critical roles in regulating the development of myeloid and dendritic cells and the activity of a number of genes, such as IL-12 and iNOS, involved in innate responses. Much of the activity of IRF8 in vitro was previously shown to require its ability to heterodimerize with PU.1 and other transcription factors to mediate transcriptional activation or repression. An in vivo test of this model was provided by studies of BXH2 mice that identified a point mutation in IRF8 in the domain required for heterodimerization. It was shown that mice bearing this mutation were very similar to those bearing a null mutation of the gene, but that the null and point-mutant mice differed in their patterns of dendritic cell maturation. This indicated that most, but not all in vivo activities of IRF8 are dependent on its ability to dimerize with other transcription factors. Previous studies demonstrated that IRF8 is expressed to varying extents in cells of bone marrow origin. To permit examinations of IRF8 expression on a single cell basis, we generated an IRF8 reporter mouse that expresses an IRF8-EGFP fusion protein under the control of normal endogenous IRF8 regulatory sequences. Expression levels were found to vary widely during various stages of hematopoietic differentiation with hematopoietic stem cells expressing little if any while dendritic cells expressed very high levels. Importantly, examining levels of IRF8-EGFP expression made it possible to define three subsets of what was previously thought to be a homogeneous population of granulocyte-myeloid progenitors. These findings provide new insights into the dynamic heterogeneity of developing hematopoietic progenitors. Studies of a mouse model of multiple sclerosis showed that development of disease was completely dependent on expression of IRF8. We found that expression of IRF8 in antigen-presenting cells promoted disease onset and progression through multiple mechanisms. These included induction of a cytokine environment that furthered the development of Th2 and Th17 inflammatory cells. IRF8 also activated microglia and exacerbated neuroinflammation. These studies provide a basis for understanding the role of IRF8 in the pathogenesis of multiple sclerosis. We also contributed to studies examining the mechanisms governing the differentiation of Th17 inflammatory cells. Activation of CD4 T cells led to expression of inducible nitric oxide synthase (iNOS). The importance of iNOS in controlling Th17 cells was shown by the marked increases in these cells in mice with a knockout of this gene or in mice treated with an iNOS inhibitor. These results indicated that nitric oxide derived from iNOS played a negative role in the regulation of Th17 cell differentiation. We also studied the transcriptional network involved in the development of Langerhans cells (LC). It was shown that IRF is totally dispensable in this process. These studies also identified a dual molecular network underlying LC differentiation and showed the central role of PU.1 in these processes.